Engine overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, chief amongst them the duration of the overspeed and the speed attained. With some engines even a momentary overspeed can result in greatly reduced engine life or even catastrophic failure.
One type of engine that is proned to overspeed is the diesel engine. Unlike a gasoline engine, where the engine speed is regulated by a butterfly valve controlled by the throttle mechanism, a diesel engine's speed is generally controlled by varying the supply of fuel.
In many environments having flammable or explosive vapors or dust, including oil fields, mines, chemical plants and in some instances vehicles, diesel engine overspeed is a real concern. In these environments, such a fuel source is drawn into the air intake of the diesel engine, resulting in uncontrollable engine speeds, also known as a runaway condition. In particular, the engine reaches a point where it can operate uncontrollably even though the engine's own fuel supply has been shut off.
In vehicles, oil mist from the crankcase can be blown into the air intake via the conventional crankcase breather pipe. The unwanted oil mist can also come from failure of the oil seals in a turbocharged diesel engine, from overfilling the crankcase with oil, or certain other mechanical problems such as a broken internal fuel pipe. In vehicles or installations that use both diesel engines and bottled gas, a gas leak into the engine room could also provide fuel for a runaway condition via the engine air intake.